The use of multiple antenna systems, such as array antennas, in radio communication applications is of interest because of the increased performance achieved with these systems. These systems are also called MIMO systems (Multiple Input—Multiple Output), i.e. multi-channel systems.
Methods utilizing multiple antenna systems for implementing time and space diversity have been developed to achieve as high a link gain as possible. One such method is the Space-Time Transmit Diversity method (STTD). In the space-time transmit diversity method, a signal is transmitted continuously to a subscriber terminal using at least two different antennas. The signal transmitted via the different antennas is different. There are two ways to implement the dissimilarity: time-space trellis codes and time-space block codes. In trellis coding, a trellis diagram is used, wherein each possible state and branches from it to other states are expressed by two symbols. When the initial state of the trellis is known, the bits to be coded can be expressed in the trellis diagram by symbols indicating the transitions to be made between different levels. The obtained symbols are then distributed for transmission over different antennas.
The bits to be coded in space-time block codes are divided for example into two-bit sequences, of which the symbols to be transmitted are generated so that the symbol to be transmitted over a first antenna is generated from a first bit and the complex conjugate of a second bit, and the symbol to be transmitted over a second antenna is generated from a second bit and the complex conjugate of the first bit.
The third and subsequent generations of mobile systems provide a chance to adapt the transfer rate according to the information to be transferred: speech is transferred at a lower rate, data at a faster rate and multimedia applications use the highest transfer rate. High transfer rates usually also require high transmission power in order to ensure the required quality on the connection. However, performance on the transmission path is affected not only by compensation for the effects of a multipath propagation environment, but also for example the spectral efficiency of the transmission, i.e. the achievement of the required quality at as low a use of radio resources as possible, typically at as low a transmission power as possible. Besides requiring less current, low transmission power also interferes less with other system signals.
The problem is to find as good a spectral efficiency as possible in multiple antenna systems, particularly at high transfer rates, for example in packet data transfer in the Internet.